A Simplified Guide to the NTSC Video Signal

This page describes the basics of the television signal,
how to analyze it on a Waveform Monitor or Vectorscope and
how to set up a monitor to color bars.

In North America the video signal is a standard, called NTSC, and is not
the same as computer, or RGB video. To maintain consistent image quality,
it is important to use only properly calibrated monitors to evaluate and
test the video picture.

Video is a linear medium like audio, unlike photography or film. A film
camera captures the entire frame of a picture in a single instant. But
video was originally designed to be transmitted over the air. Images must
be broken up and transmitted or recorded as a series of lines, one after
the other. At any given millisecond, the video image is actually just a
dot speeding across the face of the monitor glass. In the (1920's) some cameras actually created the image
by scanning the subject in front of the lens with a dot of light!

Television is also primarily an analog medium. The system we use today
was established in the 1930's. Color was grafted onto it in the early '50s.
This is when American television picked up its name, NTSC. The initials
stand for National Television Standards Committee. This is the committee
that established the standards for our television system.
The problem is that NTSC is an analog system. In computer video, colors
and brightness are represented by numbers. But with analog television,
everything is just voltages, and voltages are affected by wire length,
connectors, heat, cold, video tape, and on and on. This is why most engineers
today claim that NTSC stands for Never Twice the Same Color!

There are two other systems used for broadcasting standard definition
signals, They are PAL, and SECAM. PAL, like NTSC, is also use for television
recording, while SECAM is a transmission only standard. Different
countries use different standards.

There is a way to maintain consistency of the television signal. Because
everyone sees the world a little differently, we can't just adjust the
monitor or the tape player until the picture looks right. Over the years
a test signal has been developed, called "COLOR BARS", that functions as
a standard unvarying image that can be used to make sure pictures are displayed
consistently. This signal is generated by professional video cameras, and
by color bar generators. It is laid down at the beginning of tapes so that
those tapes can be played back later with accurate color.

Color bars are also used to align cameras in multiple camera studios
so all the cameras match. But they are only observed to align the signal
coming from the camera, and not to adjust the picture from the sensor chips.

When displayed on a monitor, color bars enable us to set up the monitor
so that it matches all other monitors. The first step in establishing a
reliable picture reference is to align picture monitors.

The term SMPTE Color Bars refers to a specific type of color
bars, the most common you will see. They are arranged to make it easy to
align a monitor that has a blue gun only switch. An older type of bars
is called
full field bars, and is now generally only found on the
cheapest pro cameras. SMPTE stands for the Society
of Motion Picture and Television Engineers. They are responsible for
setting most of the video signal and tape standards, in addition to film
standards.

PICTURE MONITORS:

Picture monitors are essentially the same as a home television receiver,
except they lack a tuner for pulling in over the air broadcasts. A picture
monitor also differs from a computer display. A computer display cannot
decode the composite video signal that comes out of the single wire from
a TV camera. It needs at least three separate wire inputs for each of the
three primary colors. That is the primary difference between computer and
television monitors. The television monitor has a video decoder that can
take the single composite color signal, turn it into the three separate
color signals and feed those to the color tube.

The adjustments on a monitor-- hue, brightness, saturation, and contrast,
only affect the picture on the monitor. They have no effect on the image
that is recorded. Hue actually rotates the phase of the color wheel. Saturation
dials color into and out of the black and white picture that is the foundation
of a color picture. Brightness (or picture) actually raises and lowers
the black level of the image. Contrast varies the video level.

First you set the level for the room/environment you're in. Turn the contrast or picture level control so that the white bar on the left looks almost white, just above gray. The White square at the bottom left, should look white, as it is 100% video, but it shouldn't bloom or streak

If you are setting up to true Split field SMPTE Bars in the bottom right of the monitor, you will see two slender vertical bars in the middle of the horizontal black bar. If you don't see them, and the bar is a solid black, turn the brightness up till you see both of them. Then turn the brightness back down slowly, so the slightly darker one on the left just dissapears in the black, and the sligtly brighter vertical bar on the right is just visible. The dark bar is 4% video, the light bar is 10% video, and the horizontal black bar they're bisecting is at 7%. Since NTSC isn't supposed to have detail in the range below 7%, the 4% bar just disseappears when set correctly. The 10% bar is just barely visible because it is only 3 units above 7.

BLUE SCREEN or BLUE GUN SWITCH

Professional monitors have a special switch, that, when depressed, turns
off the red and green guns of the tube. This causes only blue to show on
the tube. The effect on color bars is to create alternating bright and
dark vertical columns. All the dark columns should be equally dark and
all the bright columns should be equally bright. If the two outer bright
columns don't match, then the chroma/saturation control on the monitor
is turned until they do. For the inner bright columns, the hue/tint or
phase control is turned until they match.

Sony has, in the past few years, gone to a system that turns the tube
monochrome when the blue gun only switch is depressed. Adjustments are
made the same way to hue and chroma controls, but the image looks gray
and black, rather than blue.

THE WAVEFORM MONITOR:

The Waveform monitor is an oscilloscope that has been custom configured
for television monitoring. It is used to measure the voltage of the signal
and to check that all the pulses and scans of the signal are occurring
at the proper times. Our primary use for it in studio production is to
monitor the signal levels of the picture. These levels must not exceed
100% percent level on the waveform monitor, nor may they drop below 7.5%.
Signals that are too high will clip and look like white blobs. Signals
that are too low will be completely black. Generally, face tones fall in
the 70% range. White with slight detail in it will be around 90-100% and
shadowed areas will be under 30% on the scale.

THE VECTORSCOPE:

The Vectorscope is another specialized oscilloscope. Its task is to measure
color information. In a television signal color is encoded into the main
signal with a subcarrier. It is the color information on this subcarrier
that is measured by the vectorscope. It is displayed in a way similar to
the color wheels you may remember from art class. Instead of measuring
brightness of color, it indicates saturation and hue. The center of the
wheel is neutral, the closer a color is to the wheel's center, the less
saturated (or closer to white) it is. The farther out a color is, the more
saturated (less neutral) it is. A color can be dark and very saturated
or light and unsaturated. And a black image will make a dot right in the
center, as will a white image.

But brightness levels will not show on the vectorscope. Brightness can
only be measured on the waveform monitor.
Today It is quite common for the waveform monitor and vectorscope to
be combined into single unit that can switch between the two functions.
Some units even allow for the two functions to be superimposed.

TIME BASE CORRECTORS.

Video Cassette Recorders are unable, on their own, to play back a signal
that is stable enough to transmit, or cut into another video signal without
break up. A TBC is used to synchronize the tape machine with the other
signals in the studio. All the cameras in the studio, the character generator
and the special effects generator are all synchronized or genlocked to
a master sync generator. If they aren't, the picture will break up and
roll every time a cut is performed, and dissolves and fades would be impossible.

TBC's have an additional function. They are used to adjust the color
and video levels of the tape playback. The four controls used are basically
the same as those found on a monitor.

Chroma/Saturation, Hue/Phase, Brightness/Black level and Contrast/Video
level. Color bars are used here also but the adjustments are made by observing
the scopes, not the monitor.

Most professional tapes are recorded with at least 30 seconds of color
bars at the head of the tape. These reference bars are viewed first on
the waveform monitor.

Video level is adjusted so that the tallest bar just touches 100%. Black
level is adjusted so that the black bars hit at 7.5%, which is a dashed
line just below the 10% line. Next, adjust Chroma level until the large
solid rectangular column on the left just touches the 100% line. Then the
scope is switched to the Vector mode. On the vector display, 6 small boxes
are illuminated. These are labeled R,G,B and Y,C,M, the primary colors,
and their complements.

When color bars are displayed, the usual mushy blob is replaced by six
sharp dots. These dots are supposed to fall into the boxes. Accomplish
this by rotating the phase on the TBC until the dots line up with the boxes.
(A hint: You'll also see two dimmer dots much closer to the center. These
make a perfect 90 degree angle to each other. Orient them like this sideways
vee < and you'll be in the ballpark.)

Signals from outside the studio also need to be aligned with the master
studio sync generator. This is usually done when a remote camera or satellite
feed is being used. The process is the same as for the VCR, and all adjustments
are done to the TBC, not the remote camera. For this a synchronizer is
used. In the past, this unit was separate from the TBC, but now they are
most often combined into one.

Books about Television Technology
and Production:

The
Technique of Television Production This very comprehensive book
is possibly the best overview of technical television. It has been updated
for the 90s, and is handy as reference, or as a textbook. It also covers
the roles and interactions of the crew, scripting, and planning.

"The NTSC Pennant illustrates the NTSC concept of transmitting full color in large areas, two-color information (orange-cyan I axis) in medium sized areas, and only luminance information in small details"